This presentation includes mechanism of excretion, ultra filteration, Reabsorption, secretion into kidney. Formation of urine. as well as introduction of osmoregulation and mechanism in aquatic fishes including fresh water fish, marine fish, eusturine fish and migratory fish.

1. Excretion and Osmoregulation
Jigar V. Patel
Lecturer
Department of Zoology
Government Science Collage, Vankal
Ta. Mangrol, Di. Surat-394430
Email. pateljigar1818@gmail.com

2. Excretion
 Excretion may be define as the separation and elimination of the
nitrogenous metabolic wastes from the body. The waste
eliminated are called excretory products.
 Metabolism produces a great variety of substance in addition to
energy. Some of these metabolic byproducts and end products are
reutilized for metabolic activities. But certain byproducts and
products of metabolism are toxic and they will poison the body
tissue if retained. Hence these substances are elinimated.
 The organs concerned with excretion are called excretory organ.

3. Excretory organs
• Contractile vacuoles – Protozoa.
• Protonephridia or flamecells - Platyhelminthes
• Protonephridium- solenocytes– Polychaetes
• Metanephridium - Annelids
• Malpighian tubules – Insects , Arachnids
• Green glands – Crustacea
• Kidney – Mollusca
• Pronephric – Myxine
• Mesonehric - Petromyzon,Fishes and Amphibians
• Metanephric kidney – Reptiles, Birds and Mammals.

4. Nephron/Uriniferous tubule
 Nephrone is the structural and functional unit of the kidney. It is
also called uriniferous tubule.
 A nephron consists of a twisted tubule closed at one end, open at the
other end and a network of associated blood vessels. Each kidney of
man is formed of about 1 to 1.3 million nephrons.
 Each nephron has a length of 4 cm. The total length of all nephrons
(2 to 2.6 millions) will be about 65 km.
 The number of nephrons decreases after 45 to 50 year at the rate of
1% ever year.
 Human kidney has two types of nephrons. They are:
1.Cortical nephrons- The Malpighian corpuscles are in the outer
Cortex. 85% of nephrons are this type.
2.Juxta-medullary nephrons -The Malpighian corpuscles remain in
the inner cortex near the medulla.

5. Nephrone

6.  Each nephron consists of two parts namely:
1. Malpighian corpuscle
2. Renal tubule.
 The Malpighian corpuscle is a globular structure located at one end
of the nephron. It is also called renal corpuscle.
• It is the filtration unit of the nephron.
 It is located in the cortex of the kidney.
 The Malpighian corpuscle is formed of two components, namely
1. Bowman’s capsule
2. Glomerulus.
 Bowmans capsule is a double-walled cup. The inner wall is called
visceral layer and the outer wall is called parietal layer. The visceral
layer contains pores.

7.  The space lying between the two walls is called capsular
space. The capsular space of the Bowman capsule is
continuous with the lumen of the tubule.
 The Bowman capsule can be compared to a funnel with a filter
paper.
 The cavity of the cup contains a network of capillaries called
glomerulus.
 The glomerulus receives blood through a small arteriole called
afferent vessel.
 Similarly, the blood comes out from the glomerulus through
another arteriole called efferent vessel.
 The glomerulus and Bowman capsule are together called
Malpighian corpuscle. The Bowman capsule leads into the
tubular portion.

8.  The renal tubule is formed of three components.
1. Proximal convoluted tubule
2. Henle 's loop
3. Distal convoluted tubule.
 The proximal portion of the tubule arising from the Bowman’s
capsule is thrown into many coils called proximal convoluted
tubule.
 It leads into an U shaped portion called Henle's loop. It has
three region namely a proximal descending limb, a middle hair
pin bend and a distal ascending limb.
 The descending limb is made up of two components namely,
Thick descending segment and Thin descending segment.
 Thick descending segment is the direct continuation of the
proxaimal convoluted tubule.

9.  The thin descending segment connects the thick descending
segment and the hair-pin bend.
 The ascending limb is made up of two components namely,
1. Thin ascending segment
2. Thick ascending segment.
 Thin ascending segment is the continuation of hairpin bend
and it leads into the thick ascending Segment of ascending
limb.
 The thick ascending limb continues into the distal convoluted
tubule.
 A group of collecting ducts unite to form a duct of Bellini.
 The ducts of Bellini open into a pyramid. The pyramids opens
into the minor calyx.
 Three or four minor calyces unite to form a major calyx.

11.  The major calyces open into the pelvis. The pelvis leads into
the ureter. The ureter opens into the urinary bladder. The
urinary bladder opens out through the urethra.
 The Henle's loop alone remains in the medulla. The remaining
portions of the nephrons remain in the cortex.
 Each Kidney receives arterial blood through a renal artery and
the venous blood leaves the kidney through renal vein.
 The artery divides and redivides to form arterioles.
 Most of these arterioles supply the glomerulus as afferent
vessels. They leave the glomerulus as efferent vessels. The
efferent vessels, after leaving the glomerulus break up into
small capillaries which envelope the whole uriniferous tubule.
 The blood vessels the converge and form a system of veins
that merge to form renal vein by which blood leaves the
kidney.

13. Formation of Urine
 Urine is the excretory fluid eliminated by the kidney.
 Formation of urine is a highly sensitive and complex process.
 It involves three steps. They are,
1. Ultrafiltration
2. Reabsorption andd
3. Secretion

14. 1.Ultrafiltration
 The straining of blood by the Malpighian corpuscle for minute
particles is called ultrafiltration. It is the first step in urine formation.
 Malpighian corpuscle functions as the filtering apparatus. The
Bowman capsule is like a funnel with filter paper. The glomerulus
provides the blood for Filtering.
 The blood and the capsular space of the Bowman capsule are
separated by capillary endothelium, a basement membrane and
capsular epithelium.
 Arterial blood flow in the glomerulus. This blood is filtered by the
Bowman capsule and it enters the capsular space.
• The fluid present in the capsular space is called glomerular filtrate.

16.  The glomerular filtrate exactly resembles a cell free and protein free
blood.
 The constituents of glomerular filtrate remain in the same ratio as
those of the blood.
 In 24 hours, 180 litres of glomerular filtrate is formed. ie. 125ml/
minute.
 Ultrafiltration is facilitated by the following factors:
• Pores present in the Malpighian corpuscles.
• Blood pressure.
• Renal blood flow
• Osmotic pressure.
• Hydrostatic pressure.
• Constriction of glomerular arterioles.
• Sympathetic stimulation.
• Hormonal factors.

18. 2. Reabsorption
 Reabsorption is the intake of useful substances into the blood
from the glomerular filtrate.
 Every day about 180 litres of glomerular filtrate are formed.
But a normal man excretes only 1 to 1.5 litres of urine, i.e
about 1% of the glomerular filtrate.
 The remaining bulk i.e 99% of glomerular filtrate is
reabsorbed into the blood.
 Reabsorption is selective reabsorption because useful
substances are reabsorbed and the wastes are retained in the
glomerular filtrate.

20.  The useful substances of glomerular filtrate are reabsorbed into the
blood by the way of the capillary network enveloping the uriniferous
tubule.
 Reabsorption is the function of the renal tubule. The following
substances are reabsorbed from the glomerular filtrate of the
uriniiferous tubule:
1. The amino acids, glucose, protein and phosphate are reabsorbed in
first part of the proximal tubule.
2. Sodium chloride and bicarbonates are absorbed along the proximal
and the distal tubule.
3. Potassium is reabsorbed in the proximal tubule.
4. Water is reabsorbed from the distal tubules and collecting duct.
5. Sodium is reabsorbed from the ascending limb.
 The rate of reabsorption varies from 100% to negligible amount.
1. The reabsorption of glucose is 100%.
2. The reabsorption of water and sodium is 99%.
3. Urea, uric acid, creatine, etc. are reabsorbed in negligible amount.

21.  As the renal fluid moves into the collecting duct, the renal
fluid is called urine.
 At the end of duct, the urine is more concentrated than the
original glomerular filrate and is also hypertonic to plasma.
 Thus out of the 180 litres of glomerular filtrate about 179 litres
are reabsorbed.
 Water reabsorption occurs by osmosis which is a passive and
non-energy requiring process.
 The concentration of certain substances in the final urine is
higher than that present, But the absorption of glucose, amino
acids and vitamins is an active process.

22. Secretion
 Secretion is the release of unwanted materials from the blood into the
nephron.
 The concentration of certain substances in the final urine is higher than
that present in glomerular filtrate. Again urine contains certain
additional substances which are not present in glomerular filtrate. This
shows that glomerlar epithelium secretes some substances into the
lumen of the urinary tubules.
 This secretion mainly occurs in the convoluted tubules.
• Aminohippuric acid, an excretory product is secreted into the proximal
convoluted tubule.
• K+ and H+ ions are secreted by the distal convoluted tubules.
• Potassium, ammonia, urea are also secreted into the tubules.
• Creatinine and phosphates are other substances secreted.
• Moreover, a number of foreign substances introduced into the body for
therapeutic or diagnostic purposes are also removed from the plasma
mainly by the tubular epithelium. Such substances include penicillin,
phenolsulphonaphthalein, etc.

23. Hair-pin Counter Current
Multiplier Theory
 This theory was proposed by Wirz (1951) and Bray (1960). It
explains the formation of concentrated urine. This system
operates in the hairpin-like loop of Henle.
 In the descending and ascending limbs, the renal fluid flows in
the opposite direction and hence the name counter current. The
mechanism of this theory to produce concentrated urine can be
summarised as follows:
• The entire length of the nephron is permeable to water, with
the exception of the ending limb of the loop of Henle.
• Sodium is actively pumped from the ascending limb to the
interstitial fluid of medulla.

25. • The sodium pumped out of the ascending limb passes into the
descending limb by simple diffusion. The sodium again passes
into the ascending limb through the loop of henle.
• As a result of this recirculation of sodium, the loop of Henle
and the collecting duct are constantly bath into highly
concentrated fluid. As a result when the renal fluid passes
through the collecting duct more water diffuses out from the
collecting duct.
• In this way, the body is able to excrete a far more concentrated
urine. The concentrated urine is produced by the counter-
current exchanger mechanism is available only in birds and
mammals

26. Salient Features of Hair pin
Counter Current Multiplier Theory
1. The ascending limb actively transports Na+ into the medullary
tissue. Thus the surrounding medullary tissue becomes
concentrated.
2. As the medullary tissue is concentrated, water diffuses out
from the descending limb until the fluids in the descending
limb and medullary tissue are isotonic. This makes the urine
increasingly hypertonic as it passes towards the ascending
limb. The maximum concentration is 1400 millions moles/litre
at the hair-pin curve of the loop. The ascending limb is
impermeable to water.

27. 3. A certain amount of Na+ ions enter the descending limb by
diffusion.
4. The renal fluid is progressively concentrated as it flows down
the descending and then is progressively diluted as it passes up
the ascending limb.
5. The dilutron is brought about by the diffusion of Na+ from the
renal fluid to the outside. At any given Glucose, albumin, bile
salts, bile pigments and ketone bodies appear in the urine
during pathological conditions.

28. Osmoregulation
 Osmoregulation is a dynamic process where animals maintain
a suitable internal body fluid by regulating the movement of
water and salts between the body fluid and the external
medium. . . .
• Osmoregulation is a typical exainple for homeostasis. It is an
adaptation.
 Claude Bernad stated that "constancy of internal medium is
essential for an independent life".
• The term osmoregulation was coined by Hober in 1902.

29. Osmosis
 The movement of water molecules through a semipermeable membrane
from the region of higher water concentration (so lesser solute
concentration) to the region of lesser water concentration (so higher solute
concentration) is called osmosis.
• The process of osmosis can be illustrated with the help of a thistle funnel
experiment as given in the figure.
• The beaker contains distilled water and funnel contains salt solution or
sugar solution (water plus a solute such as salt or sugar). A semipermeable
membrane is tied across the mouth of the tube.
• Water molecules move from the beaker through the membrane into the
solution.
• Hence the solution level in the funnel rises until equilibrium is reached.
That is until the concentration of water molecules on both sides becomes
equal.

31.  The movement of water depends on the amount of solute it contains.
The height indicated by the rise in the water level in the funnel
represents the osmotic pressure.
• The osmosis is of two types.
• They are exosmosis and endosmosis.
1. Exosmosis
• When the body fluid has lesser solute concentration and more water
concentration, the water molecules move out of the body. This
process is called exosmosis.
Eg. Marine fishes.
2. Endosmosis
• When the body fluid has more solute concentration and less water
concentration, the water molecules move into the body from outside.
This process is called endosmosis.
Eg. Freshwater fishes .

32. Types of Medium
• As far as osmoregulation is concerned animals live in three types of media. They are isotonic,
hypotonic and hypertonic.
1. Isotonic Medium
• When the two solutions have the equal number of dissolved particles, the two solutions are
called isotonic. There is no movement of water across the membrane.
• Hence there is no osmotic pressure. Eg. Body fluid of marine protozoans and seawater
endoparasites.
• An isotonic solution is one in which a cell or organism does not change its volume.
2. Hypotonic Medium
• The solution that has lesser number of solute molecules is called hypotonic medium. It has
low osmotic concentration. Eg. Body fluid of marine fishes has lesser solute concentra tion
when compared to seawater.
3. Hypertonic Medium
• The solution that has more number of solute molecule is called hypertonic medium. It has
high osmotic concentration.
• Eg. The body fluide of freshwater fishes

33. Osmoregulation in freshwater
 Fishes are aquatic vertebrates. They live in freshwater,
seawater and brackishwater.
 Fishes are homeosmotic animals. They maintain a constant
concentration of their body fluid.
 They are osmoregulators because they regulate the
concentration of their fluid by the exchange of fluids and salts
between the body fluid and external medium.
• They are osmotically stable because they maintain a stable
internal medium.
• They are euryhaline animals because they can tolerate wide
changes in salinity.
• They develop different mechanisms to maintain a suitable
internal medium.

34.  Osmoregulation in fishes can be discussed under the following
headings:
1. Osmoregulation is freshwater teleosts.
2. Osmoregulation in marine teleosts.
3. Osmoregulation in migratory fishes
4. Osmoregulation in marine elasmobranchs
5. Osmoregulation in freshwater elasmobranchs
1. Freshwater Teleosts:-
 The body fluid of freshwater teleosts fish is hypertonic. The
freshwater is hypotonic. Hence endosmosis occurs. As a
result, freshwater enters into the body fluid and the volume of
body fluid increases.

36.  Excess of water is removed in the form of urine by the
glomerular kidney.
• Along with urine some amount of salt is also lost. Hence the
salt content of the body fluid decreases.
• This salt loss is made good by the absorption of salts from the
freshwater by the chloride cells of the gill.
2. Marine Teleosts
• The body fluid of marine teleost is hypotonic and the
seawater is hypertonic. Hence exosmosis occurs.
• The loss of water is compensated by drinking the seawater.
• As the seawater contains large amount of salt, the salt content
of the body tends to increase. The excess of salt from the body
is secreted out by the chloride secretory cells of the gills.

38. • The marine animals live in a hypertonic medium. Hence, they
face the problem of exosmosis and water moves out through
the exposed semipermeable surface like gill membranes, skin
etc. As a result marine animals face osmotic dehydration.
• This dehydration occuring in marine fishes is called
physiological dehydration.
• It makes the fish dry in seawater.
• The kidney of marine teleost is aglomerular. It reabsorbs
water from the urine hence it prevents the loss of water in
the urine. As a result the urine is concentrated.

39. 3. Migratory Fishes
 There are two groups of migratory fishes, namely anadromous and
catadromous.
• These fishes face dual problem in osmoregulation since they are compelled
to live in freshwater as well as saltwater.
1. Anadromous Fishes: These fishes migrate from the sea to the
freshwater eg. Salmon.
 When it is in seawater, the body fluid is hypotonic and the seawater is
hypertonic Hence exosmosis occurs.
• The loss of water is compensated by drinks of saltwater. Hence the salt
concentration of the body tends to increase. The excess of salt is secreted
out by the chloride secretory cells.

40.  When the fish is in the freshwater, the body fluid is hypertonic
and the freshwater is hypotonic. Hence endosmosis occurs.
The excess amount of water is removed by the glomerular
kidney in the form of urine. . . .
• Some amount of salt is also lost with the urine. This salt loss is
compensated by the chloride cells of the gills.
2. Catadromous Fishes: These migrate from freshwater to
sea for spawning. Eg. Anguilla (Eel) .
• When it is in freshwater the osmoregulation resembles that of
freshwater teleost and when it is in seawater, the
osmoregulation resembles that of marine teleost.

41. 4. Marine Elasmobranchs
 Marine elasmobranchs retain urea and trimethylamine oxide ( an
amino acid) in the blood and body fluids. Hence the concentration
of body fluid is increased more than that of marine teleosts and it is
isotonic to seawater. Hence in marine elasmobranchs no osmosis
occurs and there is no loss or gain of water.
• However, they gain salt through diffusion and by eating salt rich
marine food. This excess salt from the body fluid is secreted out by
the rectal glands present in the gut.
• The kidney is glomerular and they are unable to remove salts. But
they remove large amount of isotonic (dilute) urine. There are no
chloride secretory cells.
• The kidney reabsorbs urea from the urine. The urea is retained in
the body fluid and hence the body fluid becomes isotonic to
seawater. This prevents osmotic dehydration.

42. • Pristis and Carcharias gangeticus are freshwater sharks.
• They were descended from marine ancestors. Since they too
retain urea to some extent.
• So the body fluid is hypertonic and the freshwater is
hypotonic. Hence freshwater elasmobranchi gain water by
endosmosis and lose salts by diffusion.
• Compensatory mechanisms are like those of freshwater
teleosts.
• Kidney removes large quantities of dilute urine

43. Refrences
• Animal Physiology, Saras Publication.